Although the present invention will be described with particular reference to implantable cardioverters-defibrillators, it is to be appreciated that the invention has applications with respect to high density electronic component packaging for many other applications that require ultra-high density and high reliability in a small package.
Modern ICD's, for example, are designed to be implanted into the human body to provide pacing stimulation pulses and high voltage shocks to the heart. Such ICD's typically include a hermetically sealed housing which contains electronic circuitry for generating the needed therapy, high voltage capacitors, and a power source. Signals into and out of the circuitry are coupled through the housing by means of feedthrough terminals which, in turn, are coupled to implantable stimulation leads.
Modern ICD's have reached a high level of sophistication relative to their ability to improve the quality of life. Current research is focused on developing “smart” units capable of providing rate responsive pacing, in addition to, electrogram storage, enhanced diagnostics and four-chamber stimulation. The electronics typically include a microprocessor, memory chips such as RAM and ROM devices, and other associated active analog and digital components, together with numerous passive components, such as capacitors and resistors. It has been common in packaging such electronic circuitry to mount the assorted active and passive components onto a rigid microelectronic substrate or printed circuit board. As efforts proceed to design these advanced biomedical devices, a constant goal of reduced product size is continuously challenged by increases in circuit complexity.
There are several key factors which dictate technology selection for packaging the electronic circuitry for implantable biomedical products. The most important prerequisite is high reliability. Pacemaker electronics are designed and tested for 99 percent survival at a 90 percent confidence level for the projected product life, which is estimated to last a minimum of five years or longer from the date of implantation. Electrical performance must also be optimized for both the particular application desired and the overall power consumption.
Reduction in ICD size translates to a smaller incision in the patient and a lighter device, in general. Battery area accounts for roughly 20% of the total ICD size, the high voltage capacitors comprise about 20%, the connector top comprises about 10%, and the electronic circuitry package comprises the remaining 50%. The electronic package is the only area, at the present time, which has the flexibility to be designed to maximize component density. Consequently, double-sided multi-layer microelectronic substrate designs are quite typical.
Typical substrate technologies which meet the above criteria to varying degrees are printed circuit boards, conventional thick film substrates, and high temperature cofired ceramic substrates. However, each of these technologies possess limitations in meeting the design objectives for today's sophisticated ICD.
Printed circuit board approaches are clearly limited in packaging density when compared to thick film or cofired substrate designs.
While conventional thick film substrates have a high reliability rate, it affords only modest packaging density, suffers from poor layer to layer dielectric isolation (resulting in a higher incidence of crosstalk between conductors), and surface planarity may suffer in multilayer designs.
Conventional high density electronic circuits are customarily based on rigid ceramic substrates. So as to increase the packaging density of the components, their size can be reduced and bonding can be performed in as space-saving a manner as possible. The disadvantage of these cofired ceramic packages is that they are limited by high conductor trace resistance and poor dimensional control.
Current ICD packaging designs have several additional shortcomings that prevent them from significantly reducing their weight, volume, density and interconnectivity with other parts of the assembly that future ICDs will demand.
First, the high voltage requirements of an ICD necessitate that the interconnect substrate have a high dielectric strength. Suitable materials typically used are low-temp or high-temp co-fired ceramics. These ceramic substrates are composed of several layers, each typically 6 mils thick, and fairly heavy compared to other laminate technologies (for example, polyimide flex circuits, PCBs, and the like). Ceramic design rules require a routing density limited to 4 mil lines and spaces. Thus, the more complex the circuit design is, the more layers that are required for interconnect, and consequently the heavier the overall device that will result.
Secondly, ceramics require tungsten conductors deposited on each layer. The exposed conductors are plated with a layer of nickel followed by a layer of gold because gold does not attach directly to tungsten. It is to the last layer of gold that components are attached and electrically connected with wire bonds. Tungsten, which has an electrical resistivity three times that of copper, requires the high voltage charging components and outputs that carry high currents to be constructed from copper conductor on a PCB or a polyimide flex. Therefore, the use of a ceramic hybrid substrate necessitates the use of additional interconnect mechanisms to connect the ceramic hybrids to PCBs and to the outputs of the device.
These interconnect techniques increase the cost and volume of the ICD. They also reduce the ease of manufacturing and increase the complexity of the ICD, and ultimately result in a thicker and heavier device thereby reducing patient comfort.
Another interconnect material currently in use in implantable devices is “flexible polyimide”, or “flex”, circuits. Historically, flex circuits have been used in pacemakers and ICD's for flexible interconnection between a main hybrid substrate and the connections needed at various angles to the device feedthroughs, telemetry coils and battery connections, etc. Current assembly technology for complex multi-chip modules, such as for an ICD, on flexible polyimide substrates has been limited to surface-mounting of passive components or “flip-chips”, (i.e., IC's that are mounted with their active side facing the substrate and attached using solder bumps).
However, surface-mounting of flip-chips is not compatible with the current commonly-used diodes, power transistors and integrated circuit (IC) chip set, because they require connections to the top and back of the die using wire bonds. Thus, in order to meet the current IC chip set and interconnect density, the optimum ICD substrate must be capable of wire bond attachment, partly because chip-and-wire mounting of components offers a very high packaging density versus conventional surface mounted components, and partly because of the availability and ease of manufacture of face-up bonded chip devices. “Chip-and-wire” may be defined as hybrid technology employing exclusively face-up-bonded chip devices, interconnected to the substrate conventionally by “flying” wires, that is, wire bonds.
It was in this context that the inventors investigated, “flexible polyimide”, or “flex”, circuits for its suitability as a primary substrate for mounting chip-and-wire integrated circuits for extremely complex multi-chip VLSI circuits. Flexible polyimide exhibits several advantages that make it attractive for an ICD substrate.
More specifically, flexible polyimide as a primary substrate has a reduced thickness and weight. The substrate can be made from ˜3 mil layers, which translates to a 50% reduction in thickness and 500% lighter assembly, and it has a higher routing density, typically, capable of 2 mil lines and spaces (twice the density of ceramics).
A flexible polyimide substrate further has a lower conductor resistance since it uses copper conductors (1.67×106 ohms-cm) compared to tungsten (5.5×106 ohms-cm). The use of flexible substrates eliminate the need for additional PCB's or other polyimide flexible interconnection for interfacing with the high voltage charging components and outputs that carry high currents.
And finally, a flexible polyimide substrate is capable of integrating other interconnections. That is, a flexible polyimide substrate integrates the functionality of a conventional rigid hybrid circuit with a conventional flex circuit to thereby interconnect the feedthroughs, battery, telemetry coil, transformers, etc. into a single component.
Unfortunately, the problem with flexible polyimide is that it tends to move during wire bonding, thereby dissipating the energy, which tends to weaken the wire bond at the wire bond pad. The conventional fixture for holding substrates during wire bonding has multiple vacuum holes that when bonding one or two bonds, can be located away from the wire bond pad, but when bonding numerous of wire bonds in close proximity of each other, as is typical in an ICD design, the vacuum holes create a very non-uniform surface that provides more opportunity to let the flexible substrate move and dissipate the energy. Because the requirements for a typical ICD includes several VLSI integrated circuits (e.g., three or more of micro-processors, RAM, ROM, I/O chips, high voltage converters, etc.) and literally hundreds of wire bonds in close proximity to one another, it is necessary to have a technique and/or substrate construction that ensures that all the wire bonds are reliable without the need to test each one.
Additionally, the final assembly must be protected from handling damage and moisture that might result in the hermetically sealed housing.
What is required is a system that affords all the advantages of the flexible polyimide with an interconnect scheme that has the interconnect density and manufacturing flexibility of wire bonding approaches.